It was a scene that Graham Greene would surely have enjoyed: our men in Havana, a billionaire peer and a top Tory politician, sipping drinks on a 150ft yacht in the Hemingway marina before holding talks with a communist regime.

But the decision of William Hague to allow Lord Ashcroft to provide flights for and then accompany him on an official foreign visit to Cuba has prompted calls for the Tories to clarify the role of the party's biggest-ever donor, whose donations are still subject to an official investigation. The disclosure raises new questions about Ashcroft's role and his close relationship with Hague. It follows previous disclosures in the Observer that Ashcroft accompanied Hague to key meetings in Washington in November.

Last night Labour and the Lib Dems expressed surprise that Ashcroft, who has extensive business interests in the surrounding Caribbean and no official frontbench role in the party, is now Hague's right-hand man, attending meetings with foreign powerbrokers as the Tories prepare for power.

Last night Chris Bryant, a Foreign Office minister, questioned the Tories' decision to meet only Cuban government officials, a move he said breached the spirit of European rules, which state that such visits should not take place until and unless until there is a freedom to meet opponents of the regime.

Chris Bryant, the minister for Europe, said Hague's meeting with official government figures was a "slap in the face" for opponents of the regime. "It seems William Hague held talks with the Cuban government but completely ignored the opposition in Cuba. It has been an accepted principle across Europe, enshrined in the EU common position, that we would only make high-level ­visits to Cuba if we were free to visit whomever we want. Hague's actions have been a real slap in the face for those who are campaigning for a more open Cuba.

"Every time he is asked, Hague is remarkably vague about Lord Ashcroft's tax status. Yet he seems to have not just used his plane, but travelled with him on countless occasions and stayed on his luxury yacht. What on earth is Ashcroft doing attending Hague's overseas meetings with foreign leaders?"

Lord Oakeshott, the Lib Dem Treasury spokesman, said Hague should have declared his stay on Ashcroft's yacht.

"Hague is covering up Lord Ashcroft's lavish hospitality by not declaring he's stayed on his yacht for the price of a hotel room. It's like Hague claiming that he does not have to declare his flights on a private jet because he has handed over the fare of a Ryanair ticket."

Hague's three-day Cuban visit began on 15 March 2009 when he flew with Ashcroft into Havana, courtesy of a private jet from Flying Lion Ltd, a company controlled by the Belize-based billionaire. Hague has flown with the company on at least 10 previous occasions.

The relationship between the two men is very close, according to Tory insiders. Ashcroft was the Conservative party's main financial backer during Hague's four years as leader from 1997 to 2001, and he was nominated by Hague for a peerage. He now runs Conservative party strategy in marginal seats, and sits in on meetings in his capacity as the party's deputy chairman.

While in Havana, they met Panama's then foreign minister, Samuel Lewis Navarro. According to Panamanian news reports from three years ago, Ashcroft and Navarro were co-directors of Panama Holdings Subsidiary Inc. Together, the group met with Cuba's foreign minister Bruno Rodríguez Parrilla for a three-hour lunch in the capital. That evening, Hague and Ashcroft retired to one of Ashcroft's two "mega-yachts".

The British embassy in Havana was surprised to receive a telephone call from Hague in which he announced he was in the country and suggested a meeting with the ambassador, Dianna Melrose.

"It was unusual, to say the least. We get very few visiting dignitaries here because of Foreign Office rules," said one embassy official. "They came over for a meeting and talked openly about their meetings with Cuban government officials. Then they went back to Ashcroft's yacht."

Hague has visited the region before, courtesy of Ashcroft's company. He stayed in accommodation provided by Navarro on a visit to Panama in 2007. It was then that Hague also visited Belize, the tax haven where Ashcroft's business has been based for nearly 30 years, as well as the Turks and Caicos Islands, where Ashcroft's bank has interests.

Ashcroft has repeatedly refused to clarify his tax status in Britain. His company, Bearwood Corporate Services, is at the centre of an Electoral Commission investigation into whether millions of pounds given to the Tories were in breach of electoral law, following allegations that the company was not "carrying on business" in Britain.

According to Hague's spokesman, he paid Ashcroft for his two-night stay in his yacht at a rate equivalent to two nights' stay in a top hotel, but declined to say how much he had paid or his method of payment. In his entry to the register of members' interests, Hague declared the flights from Flying Lion, but added: "I covered the cost of my accommodation."

One former parliamentary official last night said Hague's decision not to actually register his stay on the yacht was outside the spirit of the rules. Alistair Graham, the former chair of the Committee on Standards in Public Life, said: "The key point is whether he entered an obligation when he stayed on the yacht. I think his decision not to mention it is outside the spirit of the rules," he said.

Hague's spokesman last night defended his decision to hold talks with a Cuban minister with Ashcroft. "We are not confined as the opposition by such a protocol and actual EU governments vary greatly in how they interpret it. The purpose of Mr Hague's visit to Cuba was to engage with Cuba's leaders and assess the effectiveness of EU policy towards Cuba. His view is that the communist regime in Havana may find a degree of opening up to their country by Europe and the USA a greater challenge than the sanctions imposed in recent times."

A spokesman for Ashcroft declined to comment on his relationship with Hague or Navarro but said that the peer had no investments in Cuba, and had no plans to invest in Cuba in the future.